Chapter 8 in Introduction to Biomedical Equipment Technology: Electrocardiography

By Joseph Carr and John Brown

Biomedical Instrumentation I

Schematic Representation of

Electro-Conduction System

• SA Node• AV Node• Bundle of

His• Bundle

Branches• Purkinjie

Fibers

From Berne and Levy Physiology 3rd Edition Figure 23-25

Pathway of Electro-Conduction System of the calf heart starting at AV Node

• AV Node• Bundle of

His• Bundle

Branches• Purkinjie

Fibers

From Berne and Levy Physiology 3rd Edition Figure 23-28

Electrocardiograph (ECG)• Components:

– P wave = Atrial Contraction

– QRS Complex = Ventricular Systole

– T Wave = Refractory Period

• Typical measurement from right arm to left arm

• Also see 1 mV Calibration pulse

Carr and Brown Figure 8-1

Different Segments of ECGP wave: the sequential activation (depolarization) of the right and left atria

 QRS complex: right and left ventricular depolarization (normally the ventricles are activated simultaneously)

 ST-T wave: ventricular repolarization

 U wave: origin for this wave is not clear - but probably represents "afterdepolarizations" in the ventricles

 PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS complex)

 QRS duration: duration of ventricular muscle depolarization

 QT interval: duration of ventricular depolarization and repolarization

 RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate)

 PP interval: duration of atrial cycle (an indicator or atrial rate

Typical LeadsRA = right armLA = Left armLL = left legRL = right legC = Chest

Different leads result in different waveform shapes and amplitudes due to different view and are called leads

Cardiac Axis by different Leads

Carr and Brown Figure 8-2

Types of LeadsBipolar Limb Leads: are those designated by

Lead I, II, III which form Einthoven Triangle:– Lead I = LA connection to noninverting amp. input

And RA connecting to inverting amp. Input

– Lead II = LL connection to amp. Noninverting input RA connect to inverting input and LA shorted to RL

– Lead III = LL connected to noninverting input LA connected to inverting input

LL LL LL

Einthoven Triangle:

Note potential difference for each

lead of triangle

Carr and Brown Figure 8-3

Each lead gives a slightly different representation of electrical activity of heart

•

Unipolar Limb Leads= augmented limb leads: leads that look at composite potential from 3 limbs simultaneously where signal from 2 limbs are summed in a resistor network and then applied to an inverting amplifier input and the remaining limb electrode is applied to the non-inverting input

Lead aVR = RA connected to non-inverting input while LA and LL are summed at inverting input

augmented (amplified) Voltage for Right arm (aVR)

Lead aVL = LA connected to non-inverting input while RA and LL are summed at inverting input

augmented (amplified) Voltage for Left arm (aVL)

Lead aVF = LL connected to non-inverting input while RA and LA are summed at inverting input

augmented (amplified) Voltage for Foot (aVF)

Types of Leads

LL LL LL

Types of LeadsUnipolar Chest Leads: measured with signals from

certain specified locations on the chest applied to amplifiers non-inverting input while RA LA, and LL are summed in a resistor Wilson network at amplifier inverting inputs

LL

Wilson’s Central Terminal

• Configuration used with Unipolar Chest Leads where RA LA and LL are summed in resistor network and this is sent to the inverting input of an amplifier

Electrocardiograph Traces from different leads

Normal ECG with RA, LA, LL connected

Artrial Tachycardia with RA, LA, LL connected

Ventricular Tachycardia with RA, LA, LL connected

Variations in Chest Leads C with RA and LA connected C1

C2

C3

1st Degree block RA LA LL connected

Normal

PR wave is prolonged >0.2 sec have a prolongation of delay between atrial and ventricle depolarization

2nd Degree Block Intermittent failure of AV conduction, such that not every P wave

is followed by QRS complex

Normal

3rd Degree Block

Complete failure of conduction between atria nd ventricles. Common cause is AMI (Acute Myocardial Infarction

Normal

R Bundle Branch Block

Normal

Widened QRS complex abnormalities in R S as well as T wave Q is not as affected because the left bundle branch initiates depolarization

Other ECG Signals

• Interdigital ECG: Signal taken between 2 fingers usually for home monitoring

• Esophageal ECG: electrode placed in esophagus close to heart typically used to record atrial activity where P and R wave are used to determine position

• Toilet Seat ECG: used to detect cardiac arrhythmias that can occur during defecation

Block Diagram of ECG

ECG Pre-Amplifier

• High Impedance input of bioelectric amplifier

• Lead selector switch• 1mV calibration source• Means of protecting amplifier from high

voltage discharge such as a defibrillator used on a patient

• Amplifier will have instrumentation amplifier as well as isolation amplifier

Isolation Amplifier

• Needed for safety! Want to isolate patient from high voltages and currents to prevent electric shock where there is specifically a barrier between passage of current from the power line to the patient.

• Can be done using light (photo emitter and photo detector) or a transformer (set of inductors that are used in a step up / step down configuration)

Isolation of Signal of Patient from Power needed for safety

Typical Representation of an Isolation Amplifier

Common Mode Rejection

• Until now we assumed Amplifiers were ideal such that the signal into each terminal would completely cancel lead to complete common mode rejection

• However with practical Op Amp there is not perfect cancellation thus you are interested in what common mode rejection is.

Simplistic Example of ECG Circuit

Would like to analyze what type of common mode voltage (CMV) can be derived

Common Mode Voltage (CMV)• If 2 inputs are hooked together into a differential

amplifier driven by a common source with respect to ground the common mode voltage should be the same and the ideal output should be zero however practically you will see a voltage.

• CMV is composed of 2 parts:– DC electrode offset potential– 60Hz AC induced interference caused by magnetic and

electric fields from power lines and transformers• This noise is a current from in signal, common and ground wires• Capacitively coupled into circuit• (Other markets that work at 220-240 V will experience 50Hz noise)

Analysis to reduce noise in ECG

• Common Mode Rejection: – Instrumentation amplifier

(EX. INA128) using a differential amplifier which will cancel much of the 60 Hz and common DC offset currents to each input

– If each signal is carrying similar noise then the some of the noise will subtract out with a differential amplifier

Analysis to reduce noise in ECG

• Right leg driver circuit is used in a feedback configuration to reduce 60 Hz noise and drive noise on patient to a lower level.

Use of Feedback to reduce Noise

• Thus Vn is reduced by Gain G1• Note Book forgot V in equation 5-35

G1 G2ΣΣ

Β

Vin VoV1 V1G1

Vn = Noise

V2 V2G2

B Vo+

+ +

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oin

Derivation:

• Isolation Amplifier also will attenuate noise

• Shielding of cables further reduce noise

Analysis to reduce noise in ECG

Review of Five ways to reduce Noise in ECG

• Common Mode Rejection (differential Amplifier)

• Right Leg Drive (feedback loop to decrease noise)

• Shielding of wires

• Isolation amplifier

• Notch filter to reduce 60 Hz noise

How to overcome offset voltage

1025

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2)3(

K

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Rin

Rf

Rin

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Vin

AVout

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Instrumentation Amplifier Gain (A1,A2,A3) = Non-Inverting Amplifier A4

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K

Rin

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Vin

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• If you had 300 mV of DC offset sent through two gains of 10 and then 50 you would have an offset of (300mV)(10)(50) = 150V thus you would saturate your amplifiers and not see any of your signal

• 3V offset after first set of noninverting amplifiers goes through differential amplifier A3 which reduces the offset voltage.

Problems of offset voltage and how to correct

Other Corrections for Offset• Feedback circuit where output of A4 goes

through HPF of A5 so only responses larger than cutoff frequency pass through thus the DC offset is attenuated

R and C should be switched because this is really a LPF

Affect of High Pass Filter of A5• Feedback through HPF has a

time constant of RC• 3 Modes:

– Diagnostic Mode (most time) where

RC = 1x10-6F*3.2x106Ώ = 3.2 sec

Cutoff Freq = 1/(2πRC) = 0.05Hz

– Monitor Mode (medium time) where

RC = 1x10-6F*318x103Ώ = 0.318 sec

Cutoff Freq = 1/(2πRC) = 0.5Hz

– Quick Restore (least time) where

RC = 1x10-6F*80x103Ώ = 0.08 sec

Cutoff Freq = 1/(2πRC) = 2Hz

With Feedback the DC offset is eliminated and thus can have a gain of 50 on the 2nd Non-inverting Amplifier Stage without Saturating the Circuit

Drawn IncorrectlyR and C should be switched

High Pass Active FiltersAttenuates High frequency where

cutoff frequency is 1/(2) =1/ 2RiCi

-+ Voutput

Vinput

RiA

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Low Pass Active Filters = IntegratorAttenuates High frequency where cutoff

frequency is 1/2=1/2RfCf

-+ Voutput

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Defribillator

• A Defribillator = a high voltage electrical heart stimulator used to resuscitate heart attack victims

• When a physician applies this high voltage the high voltages and currents can cause damage to medical equipment BUT physician still needs to view ECG of the patient

• How do you protect your medical equipment from excessively voltages and currents?

Protection Devices in ECGs: Glow Lamps

• Glow Lamps are pair of electrodes mounted in a glass envelope in a atmosphere of lower pressure neon gain or a mix of inert gases

• Typically impedance across electrodes is high but if voltage across electrodes exceeds ionization potential of gas then impedance drops so you create a short to ground so vast majority of current goes safely to ground and avoids your amplifiers

Protection Devices in ECGs: Zener Diodes

• Diode: device that conducts electricity in one direction only

• Zener Diode: “Turns-On” when a minimum voltage is reached so in this configuration if a large voltage is applied (ie defibrillator) the zener diode will allow current to flow and shunts it to grounds thus current goes to ground and not to the amplifiers

Protection Devices in ECGs: Current-

Limiting Diodes

• Diode: device that conducts electricity in one direction only

• Diode acts as a resistor as long as current level remains below limiting point. It current rises above the limit, the resistance will change and the current will become clamped

• Can also use a varistor (variable resistor) which functions like a surge protector that clips spikes in voltages

Types of Defibrallitor Damage

• Defibrillator is 6X greater than normal working voltage so damage will eventually occur

• Two forms of Damage:– Both Amplifier inputs are blown thus readout is a flat

line– One amplifier input is blown so the ECG appears

distorted• Cause is from zener diodes becoming open or

from glow lamps becoming defective from an air leak, or recombination or absorption of gases

• Recommended that lamps are changed every 1-2 years or sooner if ECG is in Emergency Room

Effect of Voltage Transient on ECG

• Sometime a high voltage transient is applied to the patient (defibrillator) which cause magnitudes much greater than biopotential signal (ECG) which saturates the amplifier

• Once the voltage transient signal is removed the ECG signal takes time to recover

Example of bandwidth and magnitude of various biopotentials

ECG is approximately 1 mV and spans from DC to 500 HzBook assumes Diagnostic mode is 0.05 Hz to 100 Hz

Electro-Surgery Unit (ESU) Filtering

• While a surgeon is conducting surgery he/she will want to see their patient’s ECG

• ESU can introduce frequencies into the ECG of 100KHz to 100 MHz and with magnitudes up to kVolts which can distort the ECG

• ESU introduces:– DC offsets– Obscures the signal

• ESU needs to be of diagnostic quality thus you must eliminate higher frequencies which are noise

Correct for high frequency noise using LPF so ECG can function with ESU

RC Filters

• Low Pass Filters will pass frequencies lower than cutoff frequency of FH =1/2RC

Vs

Frequency

• High Pass Filters will pass frequencies greater than cutoff frequency of FL =1/2RC

FH

Vs

FL

Schematic of Multi-channel Physiological Monitoring System

• Figure 8-11

• Instrumentation Inputs:– Up to 12 leads to ECG– Lead 13 is for RL driver (feedback to patient and then machine needed to reduce common

mode voltage– Blood pressure– Body Temperature– Blood gases

• Buffers which are noninverting amplifiers to give high input impedance or large resistor • Wilson Network: series of resistors • Digitization of Signal• Serial data output to display

Instrumentation Amplifier using OPA621

5124

249*21

21

Rg

RfG

Differential resistors are the same thus this stage of the circuit has a gain of 1

CMR OPA621

Vin = V1 = V2

Vin

VoutVV

Vout

nsignalionofcommomagnificat

nalrentialsigionofdiffemagnificat

Acomm

AdiffCMRR

12

Frequency has an effect on CMR!

Circuit Schematic of an example of ECG

•Lead I (LA – RA) means LA is going to the noninverting input and RA is going to inverting input•Precordial are the chest leads

Block diagram of Entire ECG Circuit

Digitization of Signal

DC Offset severely affect the resolution of your signal and if DC offset is too highYou may not see your ECG SignalMore bits to A/D board (10, 12, 19, 22) the more resolution to your signal because you Can represent you signal with better resolution

Homework

• Read Chapter 9• Derive the gain equation for an instrumentation

amplifier.• What resistor values could be used to produce a

gain of 10 for an instrumentation amplifier?• Why do you use non-inverting Op Amps in the

first stage of an instrumentation amplifier?• Prove that feedback used for the right leg driver

can decrease the overall noise in your circuit.• Problem 1 Chapter 8

Schedule

• Home Ch8 due 4/4• Exam 2 on 4/11

– Material on Exam 2 Chapters 7, 2, 8, 9, studio exercises, labs, homework, class notes

• ECG design labs due 5/2• ECG team presentations 5/5 (Dr. Alvarez presenting at

conference, Florence Chua and class will grade presentations)

• Final Exam 5/13 from 2:30 to 5:00 in Colton 327 (same room that we meet)– Cumulative, anything discussed during the semester will

be on the final, more emphasis on the material not covered on Exam 1 & 2

ECG Example